1. Field
Certain aspects of the present disclosure generally relate to magnetic tunneling junction (MTJ) devices, and more particularly to sub-lithographic patterning of MTJ devices.
2. Background
Unlike conventional random access memory (RAM) chip technologies, in magnetic RAM (MRAM) data is stored by magnetization of storage elements. The basic structure of the storage elements consists of metallic ferromagnetic layers separated by a thin tunneling barrier. Typically, the ferromagnetic layers underneath the barrier (e.g., the pinned layer) have a magnetization that is fixed in a particular direction. The ferromagnetic magnetic layers above the tunneling barrier (e.g., the free layer) have a magnetization direction that may be altered to represent either a “1” or a “0.” For example, a “1” may be represented when the free layer magnetization is anti-parallel to the fixed layer magnetization. In addition, a “0” may be represented when the free layer magnetization is parallel to the fixed layer magnetization or vice versa. One such device having a fixed layer, a tunneling layer, and a free layer is a magnetic tunnel junction (MTJ). The electrical resistance of an MTJ depends on whether the free layer magnetization and fixed layer magnetization are parallel or anti-parallel to each other. A memory device such as MRAM is built from an array of individually addressable MTJs.
To write data in a conventional STT-MRAM, a write current, which exceeds a critical switching current, is applied through an MTJ. Application of a write current that exceeds the critical switching current changes the magnetization direction of the free layer. When the write current flows in a first direction, the MTJ may be placed into or remain in a first state in which its free layer magnetization direction and fixed layer magnetization direction are aligned in a parallel orientation. When the write current flows in a second direction, opposite to the first direction, the MTJ may be placed into or remain in a second state in which its free layer magnetization and fixed layer magnetization are in an anti-parallel orientation.
To read data in a conventional MRAM, a read current may flow through the MTJ via the same current path used to write data in the MTJ. If the magnetizations of the MTJ's free layer and fixed layer are oriented parallel to each other, the MTJ presents a parallel resistance. The parallel resistance is different than a resistance (anti-parallel) the MTJ would present if the magnetizations of the free layer and the fixed layer were in an anti-parallel orientation. In a conventional MRAM, two distinct states are defined by these two different resistances of an MTJ in a bitcell of the MRAM. The two different resistances indicate whether a logic “0” or a logic “1” value is stored by the MTJ.
Spin-transfer-torque magnetic random access memory (STT-MRAM) is an emerging nonvolatile memory that has advantages of non-volatility. In particular, STT-MRAM operates at a higher speed than off chip dynamic random access memory (DRAM). In addition, STT-MRAM has a smaller chip size than embedded static random access memory (eSRAM), unlimited read/write endurance, and a low array leakage current.
Magnetic structures, such as magnetic tunneling junction (MTJ) structures, provide denser memory storage. The access transistor for magnetic random access memory (MRAM) structures, however, usually dominates the cell area of the memory. As such, the MRAM density is limited based on the access transistors rather than the memory storage, which limits the scaling of MRAM structures to smaller geometries.